Single-wall carbon nanotubes (SWNTs) were reported simultaneously by Iijima and co-workers at NEC and by Bethune and co-workers at IBM as early as 1993. The structure of a SWNT has been described as a single graphene sheet rolled into a seamless cylinder (Science of Fullerenes and Carbon Nanotubes, M. S. Dresselhaus et al. Ed., Academic Press. 1996). These materials are considered commercially important for a number of new technologies and as replacement materials for current technologies.
Fuel-cell-powered ground transportation, for example, has been recognized by the U.S. Department of Transportation (USDOE) and automobile manufacturers worldwide as a desired technology and a growing interest has developed for employing carbon nanotubes for these applications. For example, hydrogen storage in a carbon-based material has the recognized advantage of overall low weight. While fuel cell technology is relatively advanced, the technology for storing the fuel, such as hydrogen, lags in the development of a viable fuel-cell-powered vehicle. The USDOE has estimated that storing approximately 6 to 7 weight percent (wt %) of hydrogen should be the benchmark for a feasible hydrogen storage technology. Hydrogen storage is the wt % of hydrogen stored measured relative to the storage system.
The physical adsorption of gases within micropores (D<2 nm) in carbon materials has been actively studied for some time. Many such microporous carbon materials have been developed with very high specific surface area, e.g., As of approximately 1000-3000 m2/g. These materials, however, tend to be disordered and have convoluted surfaces exhibiting local sp2 C—C bonding. These materials, they have been tested for hydrogen storage and are not been considered promising.
Significant H2 storage has been reported in nanofilamentary carbon at room temperature, however. For example, it has been reported that 5-10 wt % of hydrogen can be stored in bundles of single-walled carbon nanotubes (SWNT), 10-20 wt % of hydrogen can be stored in alkali metal-doped SWNTs, and also about 50 wt % of hydrogen can be stored in carbon nanofibers. Cryogenic hydrogen storage of approximately 8 wt % has also been reported using high pressures, e.g., pressures of about 100 atm. None of these reports have been confirmed in other laboratories and, in some cases, the accuracy and validity of these reports have been questioned suggesting that the published values were the result of experimental error. Hence, it is difficult to build upon the efforts of others in this developing filed of hydrogen storage technology.
Moreover, many of the proposed applications of SWNT, including nano-electronic devices, field emitters, gas sensors, high-strength composites, and hydrogen storage require reasonably pure SWNT materials. Typical synthetic methods currently produce carbon tubes having a diameter in the range of 1-2 nm and arranged in the form of bundles. One particular problem associated with conventional synthetic techniques is that the intended SWNT is a minority constituent in the reaction product. Also present, for example, are amorphous sp2 carbons which coat the fiber walls and multi-shell carbon species which cover metal catalyst impurities that result from the catalytic production of conventional carbon fibers and SWNTs. It is a challenging problem to separate the desired SWNT from its accompanying mixture of amorphous carbon impurities, multi-shell carbon species and metal impurities without adversely damaging the carbon fiber or the tube walls.
Many purification procedures have been developed to remove the inherent contaminates from carbonaceous soots produced in an effort to obtain the desired SWNT. These methods include hydrothermal treatment, gaseous or catalytic oxidation, nitric acid reflux, peroxide reflux, cross flow filtration, and chromatography.
These treatment, however, tend to chemically destroy a significant portion of the desired carbon nanotubes, require excessive production times and, in the case of arc produced carbon nanofibers, have a marginal effect in purifying the desired carbon nanofibres from its impurities, such as amorphous carbon phases and graphitic carbon phases covering metal impurities. It is also unfortunate that the results of many of these purification processes have not been even semi-quantitatively determined with respect to the purity of the final product. Thus, they have been of little aid to the skilled artisan in advancing the understanding of purification procedures thereby reducing the predictability of successfully achieving a process of purifying SWNT in high yield and throughput.
Furthermore, most of the purification processes reported previously were for carbon-nanofibers produced by a pulse laser vaporization (PLV) process which inherently produces smaller amounts of catalyst residue and smaller amounts of multi-shell carbon phases as well.
Accordingly, a need exists for the efficient purification of carbon fibers, particularly nanosized carbon fibers in high yield and throughput.